In a battery operated power supply, a DC-DC converter is, generally, employed in order to regulate a voltage from a battery.
Particularly, electric flash units employ a power supply usually wherein a large output capacitor is charged, over a period of time, to a voltage of sufficient amplitude to supply a photoflach lamp. These power supply usually employ a mechanical vibrator for the purpose of converting a relatively low battery voltage to the relatively high undirectional voltage required for a load such as, for example, the photoflash lamp. Since it is customary to ready an electric flash unit by energing it before a photographic picture is to be taken, the output capacitor of the unit is energized continuously until the voltage across it is applied to the photoflash lamp. The drain on the batteries during this stand-by operation may be considerable, and reduces the battery life. Also, the light available from the photoflash lamp decreases as the output voltage of the battery source decreases.
The power supply of this kind of the load employs a DC-DC converter which comprises a direct current power source circuit having a battery, a voltage converter circuit for converting a direct current voltage into an alternating current voltage, a rectifier circuit for rectifying the alternating current voltage to a direct current voltage.
The direct current power source circuit usually includes a power source switch which is manually operated to switch on and off a current from the battery. The voltage converter circuit is connected to the battery by way of the power source switch. An oscillator circuit is usually employed in the voltage converter circuit for obtaining an oscillation output voltage, and the direct current voltage is obtained by means of rectifying the oscillation voltage by the rectifier circuit.
This type of DC-DC converter is, generally, used in order to obtained the direct current voltage to be regulated in response to the load. Accordingly, it is necessary to invert the direct current voltage to the alternating current voltage in order to boost or drop the output voltage of the power supply. To this end, it is convenience to use substantially an oscillating transformer, a resistor, an oscillating capacitor and an oscillation switching element.
In an usual power supply, a toggle type switch or a slide switch is used as a power source switch, and therefore the power source switch continues an ON state or an OFF state, because it is manually operated. The voltage converter circuit tries to continues its operation so long as the power source switch is not turned OFF. Accordingly, if the power source switch is left in its conductive condition for a long time interval, the voltage converter circuit continues an activation so as to supply the power to the load, and, therefore, the current from the battery flows until the power source switch is opened.
During this time the power supply consumes energy from the battery without any useful result. As the battery ages its output voltage drops and the battery becomes incapable of operating.
One known method of revolving the above drawbacks, is embodied a power supply in which a timer circuit is provided between a power source circuit and a voltage converter circuit in order to interrupt the current which flows from the power source circuit, if the power supply is unnecessary to use within a given time period. The timer circuit, however, includes a semiconductive element, such as, for example, a power transistor which is serially connected to the power source circuit and the voltage converter circuit, and, as a result, the actual power input to the voltage converter circuit B is reduced due to the high and specific power loss of the power transistor. Accordingly, the above described method does not effective use of the battery.